Hi, I have created a document field on the invoice transaction to upload a file to that. Currently, it is not accepting multiple file uploads to the same document field. I have a requirement to upload more than one file. Please let me know if there is a way to do that l. Thank you in advance.

Attach the file to the transaction. Instead of a custom field. It will go into the communication tab.

NetSuite's document fields are single-file only by design. Your best option is to create multiple document fields, such as Document 1, Document 2, and so on. Alternatively, you can take a related records approach. Create a custom record type for attachments that connects to your invoice with a one-to-many relationship.

why bother just install drag and drop

@Sam-I-Am can you please elaborate on how to use this drag and drop feature on an invoice and send those invoices to the customers upon on clicking the send to customer button triggered?

Drag and drop allows you attach multiple files to a record basically does what @raghavsaid bypassing the manual upload process in communications>file tab option. To send to the customer you will need some workflow or scripting. Workflow shouldn't be that complex since action is quite straight forward.

I already have a workflow that is sending the customer invoices. But, the challenge here is that I cannot map this file tab in the workflow.